When T4 bacteriophage infects Escherichia coli it specifies the synthesis of a peptide T, which attaches to host valyl-tRNA synthetase and causes the enzyme to bind tightly to tRNA. A radioimmunochemical assay for this peptide will be used to measure its synthesis and distribution in bacteria infected with normal and mutant viruses. The associated species of tRNA will be identified by purification from the enzyme complex, chromatography on BD-cellulose and aminoacylation reactions with tritiated amino acids. The stoiciometry of the phage modified valyl-tRNA synthetase tRNA interaction will be determined using a membrane filter binding assay. The physiological consequences of valyl-tRNA synthetase modification will be assessed by measuring the rate of protein synthesis in vivo and in vitro in samples differing only in their state of valyl-tRNA synthetase modification. Sucrose-density- gradient-electrophoresis will be used to analyze the effect of T4 bacteriophage infection on the electrophoretic mobility of other aminoacyl-tRNA synthetases.